As
a lunar geochemist I have been approached many times by people who believe
that they have a sample from the Moon. Common stories are (something like)
“This dust was given to my late grandfather by astronaut Buzz Lightyear” or
“This rock that I found in my petunia pot looks just like lunar meteorite QUE 94281 on your website.” Lately, people
have been sending me reports that they have obtained of chemical analyses
from labs or one of those hand-held x-ray “guns.” So, here’s what you need to
know in order to interpret those reports.

Major Elements
– In lunar rocks and soils 99% of the mass consists of the following 7
chemical elements.

Below
are charts I’ve made from data from dozens of literature sources and my own
lab for what we geochemists call the “major elements” and “minor elements” in
samples from those 6 Apollo mission and 3 Russian Luna missions that brought
samples back from the Moon. To make it simple, I’ve stuck to just soil
(regolith) samples. I’ve also included data for those lunar meteorites that
are breccias because many to most of
these rocks are composed of lithified soil. The lunar meteorites come from
all over the Moon whereas the Apollo and Luna mission all come a small are of the nearside.

In
rocks of the Earth and Moon, oxygen is the most abundant chemical element, 41-45%
on the Moon. Practically nobody actually measures the concentration of oxygen
in rocks any more. We measure the “metals” like iron and aluminum.

Terrestrial
geochemists like to “express” the measured concentration of, say, silicon “as
the oxide.” They measure the concentration of Si and state the concentration
as the SiO2. So, 10.0 % Si is
21.4% SiO2. Quartz is a form SiO2, but quartz is
rare on the Moon. Nearly all the Si is in silicate minerals like plagioclase,
pyroxene, and olivine. Likewise, there is no actual MgO (the mineral
periclase) on the Moon; magnesium is carried mostly by the minerals pyroxene
and olivine. We express the metal concentrations as oxide concentrations
because the sum of 10 major and minor metal oxides above should be 100±1%. If
not, we’ve done something wrong (!) as there are no (= insignificant amounts
of) carbonates, sulfates, or hydrous (water-bearing) minerals on the Moon.
Lunar meteorites, however, often to contain carbonates, sulfates, or hydrous
minerals as a result of weathering on Earth after they land.

So,
for geochemists, the bottom and left axes of the plots below are in
weight-percent oxide. For scrap-yard dealers and jewelers who might have an
x-ray gun set to the “metal” setting, use the top and right axes.

All
the plots have aluminum concentrations on the horizontal axis. I do it that
way because Al varies over a large range in lunar samples. (To confuse you
even more, elsewhere here I’ve put FeO+MgO
on the horizontal axis, but that is OK because there is a strong anticorrelation between Al2O3
and FeO+MgO.)

Finally,
in the plots below, each point for Apollo 11, and the 3 Luna missions represents
a chemical analysis. For example, nearly all the Apollo 11 points represent
sample 10084 (which is probably the most well characterized geologic sample
ever). For Apollos 12, 14, 15, 16, and 17, each point represents a numbered
sample (“surface” and “trench” soils, no cores), e.g., samples 12032, 14163,
15071, 67555, and 76501 (mean of all available analyses for each). The large
spread for some of these missions reflect the compositional variation among
the various locations at which samples were collected at the site. For the
lunar meteorites, each point represents a named stone, e.g., MacAlpine Hills 88105 or Northwest Africa 8046 and its pairs. For
reference, each plot also includes an “Earth” point which is an average of 4
different estimates I found in the literature for the mean composition of
upper continental crust of the Earth.

On Earth, SiO2concentrations
in rocks vary from 0% to 100%. The variation on the Moon is much less because
the 3 major minerals in lunar rocks, plagioclase feldspar (usually
anorthite), pyroxene, and olivine all have about the same SiO2
concentration.

This confuses people. On Earth,
iron exists in the 2+ (ferrous) and 3+ (ferric) oxidation states so in
chemical analysis of rocks, Fe concentrations are usually stated as % Fe2O3
because the ferric oxidation state is more common than ferrous oxidation state.
On the Moon there is (effectively) no oxygen-bearing atmosphere so there are
no iron 3+ iron minerals. The iron in pyroxene, olivine, and iron-titanium
minerals like ilmenite is all in the ferrous (2+) oxidation state. To
complicate the issue, some of the iron in every lunar soil exists as metal.
Up to 10% of the iron in some of these sample is metallic, usually as
iron-nickel metal derived from meteorites. So, in analyses of lunar samples,
results for iron are usually stated as “total Fe as FeO” or FeOT.
The anticorrelation in this plot occurs because soils on the left (basaltic)
are dominated by the Al-poor, Fe-rich minerals pyroxene, olivine, and
ilmenite whereas those on the right (feldspathic) are dominated by the
Al-rich, Fe-poor mineral plagioclase.